1. FIELD OF THE INVENTION
The present invention generally relates to an optical density measuring apparatus for securing homogeneity of infrared measuring light which has a pair of cells being a reference cell and a sample cell in an infrared optical system thereof and which finds an optical density of a sample on a basis of a ratio between a pair of intensities of the infrared measuring lights passing through the pair of cells, and particularly relates to the optical density measuring apparatus therefor with a double-beam optical system which transforms the infrared measuring light emitted from a light source into a parallel infrared measuring light by a collimator lens, splits the parallel measuring light into two split parallel pencils of light, and makes the first pencil of light and the second pencil of light pass through the reference cell and the sample cell, respectively.
2. DESCRIPTION OF THE RELATED ARTS
A variety of types of optical density measuring apparatuses have conventionally been provided. The types of optical density measuring apparatuses are generally classified by their optical paths, or pencils of measuring light, into two types: a type with a single-beam optical system and a type with a double-beam optical system. The former type, namely, the type with the single-beam optical system, which has been conventionally popularized for a long time, has a construction in which a cell is positioned in one pencil of light travelling from a light source to an optical receiver. With the construction, the cell is filled up preparatorily with pure water as a reference liquid, and the quantity of transmitted light of the measuring light that passes through the pure water is detected beforehand by the optical receiver. Then, after replacing the pure water inside the cell with a sample, the quantity of transmitted light of the measuring light that passes through the sample is detected, and the density of the sample is found or calculated from the ratio between the quantities of both of the transmitted measuring lights.
This type of optical density measuring apparatus with the single-beam optical system, which employs the single optical path or pencil of light, has an advantage that an optical identicalness is maintained or assured at time of measuring the reference cell and at time of the sample cell, except for the density of the sample inside the cell.
However, a blank calibration (zero-calibration) of the optical density measuring apparatus belonging to this type has to be periodically executed in order to guarantee the identicalness thereof for a long term, and a user has to take a trouble to replace the sample with pure water for the calibration. As a result, this leads to a problem of reduction in measurement efficiency of measuring the optical density of the sample.
Also, an additional device or structure for the replacement of the sample with the pure water is indispensable, and this leads to another problem of high cost of the apparatus.
Furthermore, in case that even a little amount of the sample remains inside the cell when replacing the sample with the pure water at time of executing the blank calibration, the reliability of the blank calibration accuracy is impaired.
Meanwhile, there has been suggested an apparatus of the type with the single-beam optical system which employs a cell called a cylinderical type variable-length cell (refer to Japanese Laid-Open Patent Publication No. 4-1556). This cylinderical type variable-length cell changes the position of a piston therein to allow the thickness of a cylinder space, in which a sample is put, to be changed into a reference cell length and a sample cell length. This apparatus has an advantage that it necessitates no pure water for the blank calibration; however, it is very difficult to maintain the piston at a predetermined position with high accuracy for a long period. Therefore, the apparatus has not yet been put into practical use.
On the other hand, the apparatus of the type with the double-beam optical system is intended for solving the aforementioned problems of the type with the single-beam optical system, thereby obviating the need to carry out the blank calibration employing the pure water.
The type with the double-beam optical system includes a type of apparatus in which a measurement light emitted from a light source is directly split into two optical paths or pencils of light from the beginning, in which a reference cell and a sample cell are positioned in the first pencil of light and the second pencil of light, respectively, in which identical samples are put in both of the cells, and in which the lights that have passed through the cells are received by an optical detector or optical receiver (refer to Japanese Laid-Open Patent Publication No. 3-223654). This apparatus has an advantage that it necessitates no pure water for blank calibration, and an advantage that both of the cells are stationary (or fixed) and are provided with no movable portion.
However, the apparatus directly splits the measuring light from the light source into the two partial lights (i.e. two pencils of light) even though the light source is identical. Therefore, the homogeneity or identicalness of the two diverged optical paths, or pencils of light, is not guaranteed at all. Therefore, a calibration curve must be prepared and made every time the light source is aged, or every time the light source itself is changed. As is well known, the preparation and making of the calibration curve is a troublesome work requiring many hours and much burden.
Therefore, assuring the homogeneity, or identicalness, of the optical paths, or pencils of light, to be used for the measurement of both of the reference cell and the sample cell is absolutely necessary for the improvement of the accuracy of the measurement thereof.
As another conventional apparatus of the type with the double-beam optical system, which seems to somewhat solve the aforementioned problem, there can be enumerated an apparatus disclosed in Japanese Laid-Open Patent Publication No. 5-332933. The essential part of this apparatus is schematically shown in FIGS. 1 and 2.
This apparatus has an optical system including an infrared light source "O", a shutter "S" for taking part of light out from the light source "O", a mask "M" having two apertures "M1" and "M2", a collimator lens "L2", a reference cell "C1", a sample cell "C2", an interference filter (not shown), a focusing lens (not shown), and an optical receiver or optical detector (not shown). In this optical system, an infrared measuring light emitted from the light source "O" pass limitedly through a region of an opening "S1" of the shutter "S" located ahead. In the figures, the opening "S1" is located in an upper position.
In the figures, the infrared measuring light that has passed through the opening "S1" further passes through the first aperture "M1" of the mask "M", the travelling pencil of the infrared measuring light is transformed into a parallel pencil of light by the collimator lens "L2," and then the parallel pencil of light passes through the reference cell C1.
On the other hand, when the opening "S1" of the shutter "S" moves to a lower position (not shown), the infrared measuring light emitted from the light source "O" passes through the second aperture "M2" of the mask "M", the travelling pencil of the infrared measuring light is transformed into a parallel pencil of light by the collimator lens "L2," and then the parallel pencil of light passes through the sample cell "C2" (a detailed description about subsequent processing of the transmitted measuring light is omitted below).
In the optical system of the apparatus shown in FIGS. 1 and 2, the two split parallel pencils of light, which are derived from one pencil of light emitted forward from the light source "O," are used as a light that passes through the reference cell "C1" and a light that passes through the sample cell "C2", respectively, by employing at least an identical optical component. Therefore, the homogeneity, or identicalness, of both of the split parallel pencils of light is presumed to be guaranteed considerably.
However, if a measurement with ultrahigh-accuracy is desired, even this apparatus cannot realize a sufficient homogeneity or identicalness of the two split parallel pencils of light employed therein. The reason for this will be described below with reference to FIGS. 1 and 2.
It may be generally considered that, if an identical light source is used, identical fluctuations arise simultaneously in the two parallel pencils of light with respect to a fluctuation in light emission intensity of the light source. However, strictly speaking, the light source has a specified area with respect to the entrance pupil diameter of the optical system. Therefore, the improvement of the accuracy of measurement cannot be realized unless the optical system is constructed by taking into consideration the fact that the optical information of the intensity, and so on, of the infrared measuring light emitted from each luminous point constituting the specified area differs at each luminous point.
The light source "O" shown in FIGS. 1 and 2 includes a filament "O1" that serves as an illuminant. In FIG. 1, the center of the filament "O1" having a specified area, and the point of center of symmetry of the two apertures "M1" and "M2," are both located on an optical axis "P." In this construction, an infrared measuring light emitted from the point of the center "O2" of the filament "O1" is limited by the mask apertures "M1" and "M2," and then the two split measuring lights are made to pass through the collimator lens "L2" so that the two split measuring lights are transformed into two split parallel pencils of light "B1" and "B2" symmetrical relatively to along the optical axis "P." That is, in a certain measurement space of the cell "C," the pencils of light "B1" and "B2" can be regarded as lights with an identical quality.
On the other hand, the infrared measuring light emitted from an end "O3" of the filament "O1" is transformed into two split parallel pencils of light "D1" and "D2" in a similar manner, as shown in FIGS. 1 and 2. As is apparent from the figures, the pencils of light "D1" and "D2" are not symmetrical relatively to the optical axis "P", and are unbalanced. That is, if a pair of apertures on the mask are arranged on the basis of a mere simple conception that only the symmetry of the apertures relative to the optical axis is essential, then the two pencils of light "D1" and "D2" may pass through asymmetrical areas (i.e. areas not symmetrical relative to the optical axis "P") of the measuring space in which the cells "C1" and "C2" are positioned.
A further problem may arise by a positional shift of the filament "O1" when the light source "O" itself is replaced by another one. FIG. 2 shows a state in which the position of the filament "O1" is shifted from the state of FIG. 1. FIG. 2 illustrates a situation where an end "O4" of the filament "O1" is positioned on the center of the optical axis "P". In this case, the other end "O3" of the filament "O1" is largely displaced from the center of the optical axis "P." Therefore, the pencils of light "D1" and "D2" that have been limited by the mask apertures "M1" and "M2" are more largely displaced or shifted from the optical axis "P" in the measuring space.
Under an extreme situation, it is possible that the pencil of light "D2" that passes through the sample cell "C2" may include almost none of the optical information upon the end "O3" of the filament "O1."
Consequently, under such a situation, the two pencils of light have optical information with utterly different qualities when the light source is replaced. Therefore, the relation between the density and the absorbance measured or calibrated on the basis of the optical information prior to the replacement of the light source becomes unusable, which in turn requires a remake (or reconstruction) of the calibration curve with respect to the replaced new light source.
Another problem is a positional nonuniformity of optical penetration characteristics of an interference filter which is employed in this type of spectroscopic apparatus. As a spectroscopic filter to selectively allow a specified wavelength to pass the filter, the interference filter is simple, convenient and thus widely used. However, its spectroscopic spectrum characteristics are not always uniform depending on each interference filter. This nonuniformity thereof is in connection with the problem concerning a manufacturing process of a multilayer deposition film, and even the spectroscopic spectrum characteristics of a plurality of interference filters manufactured through an identical manufacturing process have variations in the peak wavelength and half width thereof. Further, strictly speaking, even in one filter, the spectroscopic penetration spectrums are not necessarily the same, depending on what part of the filter the light passes.
Since the pencil of light is single in the aforementioned conventional type of the apparatus with the single-beam optical system, the positional nonuniformity of the interference filter is similarly included both at the sample measuring stage and at the reference measuring stage, causing no such problem as described above. However, in case that the pencil of light is split into parts and that a difference in spectroscopic spectrum between both the optical paths is caused, a fatal error arises in the measurement result.
Therefore, it is an important technical object how the positional nonuniformity of the spectroscopic spectrum is allowed to be uniformly included in both of the pencils of light.